A starter for an internal combustion engine has an electric motor which drives the engine via a pinion mechanism which engages a flywheel of the engine to enable the engine to start.
Because the motor draws a large current when driving the engine, a solenoid switch is used to connect the motor to the power source, typically a battery of the vehicle. The solenoid switch includes a movable core, a pull-in coil, a holding coil, contacts and a plunger assembly arranged to connect the contacts. The pull-in coil is connected in series with the motor before it is shorted out by the contacts. The holding coil is arranged in parallel to the pull-in coil and motor. When the starter is switched on, the pull-in coil, the holding coil and the motor are energized. As the motor is energized through the pull-in coil, the motor operates on a reduced voltage. The energized pull-in coil and holding coil generate a magnetic field to move the movable core, thereby causing the pinion to engage with the flywheel and the plunger assembly to connect the contacts, shorting the pull-in coil to fully energize the motor to rotate the engine.
At the instant the solenoid switch is turned on, the pull-in coil and the holding coil both are energized to attract the movable core. Once the movable core has been retracted the force required to hold the movable core in the retracted position is less due to the reduced air gap and thus the pull-in coil can be de-energized to provide more power to the motor while the movable core is held retracted by the magnetic force generated by only the holding coil.
The use of two coils thus saves energy but the solenoid itself is expensive as both coils are made of copper and require many turns to achieve the required magnetic force to operate the solenoid reliably.